Thailand nature expedition: February 2008

Toxotes microlepis

The archerfish (or archer fish) are a family (Toxotidae) of fish known for their habit of preying on insects and other small animals by shooting them down with water droplets from their specialized mouths. A large lower jaw helps these fish to hunt. The family is a small one, consisting of seven species in the genus Toxotes; all occur in fresh, brackish, and marine waters from India to the Philippines, Australia, Louisiana and Polynesia.

Archerfish bodies are deep and compressed, with the dorsal fin set far back, and the profile a straight line from dorsal fin to mouth. The mouth is protractile, and the lower jaw juts out. Sizes are generally small, but T. chatareus can reach 40 cm.

Archerfish are remarkably accurate in their shooting; adult fish almost always hit the target on the first shot. They can bring down an insect six feet above the water's surface. This is partially due to their good eyesight, but also because of their ability to compensate for the refraction which occurs when light travels through water. They do this by swimming directly under the prey, where the distortion is the least. However, experimentation has shown that the Archerfish can hit prey at greater angles than a perpendicular angle without losing on accuracy.

When an archerfish selects its prey, it sticks its snout just above the surface and squirts a jet of water at its victim. It does this using the narrow groove that is formed in the roof of its mouth. It presses its tongue against this groove to form a narrow channel, then contracts its gill covers to force a powerful jet of water through the channel.

The resulting jet of water can be up to 2-5 m long, but their accuracy only allows them to shoot insects 1-1.5 m away. If the first shot does not knock the victim into the water, the archerfish will keep trying. Young archerfish start shooting when they are about 2.5 cm long, but are inaccurate at first and must learn from experience. During this learning period, they hunt in small swarms. This way, the probability that at least one jet will hit is higher. The fish can also alter the power of the shot for prey of different sizes.